Hello to everybody. I'am trying to represent using Ansys CFX a 10 m long duct initially filled with air. The duct has only one way in or out, a hole at the top through which water is introduced.

I was trying to simulate it using the surface model, with homogeneous model,transient, double precision,

I get two strange results:

1. When monitoring the water volume fraction in the duct I noticed that In the first time step (just when the water is invading the access hole) all the domain achieves a water volume fraction of 0.5. To me this sounds like a numerical problem, but I don't know if anyone has experienced something like this?

2. The velocity at which the water falls is more or less the same as the inlet velocity (2 m/s). Shouldn't it be close to the free falling speed of a waterfall (around 10 m/s)

some pictures below, hope the quality is enough.
The velocity at the inlet is 2 m/s.
the duct diameter is around 2"
The duct lenght is around 10 m
The Mesh is not very refined, but enough for a first approximation

I am starting to believe that the first problem is related somehow with the given initial conditions. Seems to me that the solver is imposing a 0.5 volume fraction for the entire domain, ignoring the initial conditions file.

Your first question is an initial conditions problem. Just define the IC as VF=0.

The second question is dodgy. Firstly, the flow will take some time to accelerate to terminal velocity. You will probably have to run it for longer. Secondly, do you know what terminal velocity is? It is a function of drop size, fluid density, drop shape and other parameters - and it is also a function of the surrounding air flow. Your flow is clearly different to your reference waterfall. Additionally, a waterfall will entrain an air gust with it so the water droplets see reduced drag and this will increase the terminal velocity.

Yes, I did some calculations to determine terminal speed of one water dropplet , and I got that result, somehow similar to that of a waterfall. Terminal velocity is also achieved in a very small lenght, but maybe in this case due to the trapped air, it will require longer distances.